xdp: sample: Missing curly braces in read_route()

The assert statement is supposed to be part of the else branch but the
curly braces were accidentally left off.

Fixes: 3e29cd0e

 ("xdp: Sample xdp program implementing ip forward")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>